Inmates at one of Britain’s most high security jails set up secret phone lines which allowed them to make cut price calls to people on the outside - without jail staff listening in.

The scam was eventually busted by governors and police were drafted in for an urgent probe at category A Wakefield jail - dubbed Monster Mansion, home to some of the most dangerous prisoners in the UK.

The scam - a huge security risk - came about when a lag spotted a loophole in the telephone system.

A contact on the outside set up a low charge premium rate telephone service which could be used by cons ‘dialling ion’ on a specific number using the jail’s payphones.

The service was likened to calling cards sold at newsagents for cheap international calls. Once cons were conneted to the low charge number they were prompted to dial the number of the person they actually want to speak to.

Cons pretended to jail bosses that the number was a solicitors’ firm. That meant it could be dialled out from prison telephones - and that staff were legally unable to monitor calls.

The scam allowed inmates to swerve 18p a minute call charges which ordinary prisoners have to pay when dialling friends and family directly.

A prisoner talks on the phone (Image: Getty)

And because of the secrecy it could have left cons free to make tormenting calls to victims’ or their families.

At least three prisoners are said to have set up secret lines because the con was busted.

Police were called in but an investigation is said to have ruled no criminal offences took place.

But the phone dodge was a breach of prison rules, and the offenders involved lost privileges as a punishment just

It is not known which offences the phone scammers had been originally committed. But Wakefield is home to some of Britain’s most notorious murderers, rapists and paedophiles.

The scam was coined by an inmate who was jailed for life in 2002. He detailed it on a blog he writes from jail under the name Adam Mac.

He is believed to pen the blog by sending handwritten entries to friends on the outside, who publish it online.

He revealed the phone scam had been busted in an entry on January 31, saying he had simply been trying to avoid the “extortionate” costs of using BT phones in prison, or obtaining a contraband mobile.

And the inmate confirmed he had the other unnamed prisoners involved had been punished by the governor.

In a second entry last Wednesday (Feb 8) he wrote: “I know I did the wrong thing.

“I see now that although I only wanted to reduce the cost of my-calls, the system I put in place could have enabled others to say and do anything they wanted via their unmonitored telephone access and this could have had potentially devastating consequences.”

The Ministry of Justice and West Yorkshire Police declined to comment on the incident.